Source code formatting cleanup, and adding a couple helper methods to d4 to determine if a value is defined or undefined.
Major rework to the way mixins work. Now you can pass an array of mixins to the chart,or a single feature. However, I had to change the way the mixin object is structured to explicitly require a name key instead of inferring it, this is because property order of an object is not guaranteed, and previously I was relying on the feature name being the first key.